Redox Reactions

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Redox ReactionsThe corrosion of metals, especially the corrosion of iron (rusting), has a great economic impact.

Rust is one of the common causes of bridge accidents.

The Kinzua Bridge in Pennsylvania was blown down by a tornado in 2003 largely because the central base bolts holding the structure to the ground had rusted away, leaving the bridge resting by gravity alone.

Redox Reactions

The January report by independent engineers said the rust was starting to "detrimentally affect the structure".

Rust fears for Harbour BridgeAuckland Harbour Bridge is rusting away at "structurally significant points", forcing engineers to step up maintenance work.And a cable - an "essential" part of the bridge's bracing system for earthquakes and high winds - has been disconnected to deal with more rust. Other cables can bear the weight when one is disconnected.

One of the walkways that had been used by Bridge Climb operators is corroded, necessitating safety repairs, according to an engineering report.

NZ Herald 22/03/2009

Redox ReactionsExplain why this reaction is a redox reaction!

Give more than one answer.

Gain/loss of oxygen/hydrogen, gain/loss of electrons (transfer), increase/decrease in Ox.number

Remember:• A redox reaction is any reaction involving a transfer

of electrons.• In all redox reactions, oxidation and reduction

happen at the same time.• Oxidation is loss of electrons/ increase in oxidation

number.• Reduction is gain of electrons/decrease in

oxidation number.• Oxidising agents (oxidants) are themselves

reduced.• Reducing agents (reductants) are themselves

oxidised.

Redox Reactions

Examples:Write the oxidation number for each of the underlined atoms:

[FeSCN]2+

Redox Reactions

Example:Use oxidation numbers to determine whether the following reaction is a redox reaction, and if so, which element has been oxidised, and which has been reduced:

4NH3(g) + 3O2(g) → 2N2(g) + 6H2O(l)

-3 00 -2

It is a redox reaction. The nitrogen in ammonia has been oxidised (ON increased). The oxygen has been reduced (ON decreased).

Complete worksheet #1

Colours of species in redox reactions

The colour of any species depends on the oxidation number.You are expected to know a number of species and their colours.

Reduced form Oxidised formCu brown solid Cu2+ blue aq

SO2gas SO4

Mn2+ aq H+/MnO4- purple aq

H2O2liquid O2

H2O liquid H2O2liquid

Cr3+ blue/green aq Cr2O72- orange aq

Fe2+ pale green aq Fe3+ orange aq

Cl- aq Cl2pale green gas

Br- aq Br2red/orange liquid

H2gas H+ aq

Reduced form Oxidised formMnO2

brown solid H2O/MnO4- purple aq

MnO42- green aq OH-/MnO4

- purple aq

I- aq I2 in I- = I3- brown aq

I2 in I- = I3- brown aq IO3

H2S gas S yellow/white solid

Pb2+ aq PbO2brown solid

NO2brown gas NO3

C2O42- aq CO2

S2O32- aq S4O6

Br2red/orange liquid BrO3

Names of unfamiliar ions:C2O4

2-Oxalate ion (from oxalic acid)S2O3

2-Thiosulfate ionS4O6

2-TetrathionateBrO3

- BromateIO3

- Iodate

Citrobacter live in the intestines of warm blooded animals, without oxygen. They can respire by converting tetrathionate

into thiosulfate.

Rhubarb contains oxalic acid. This makes your teeth feel furry.

Some species can be recognised by their colour.For example:The purple permanganate ion (MnO4

-) can be reduced in alkaline conditions (presence of OH-).The product is the green manganate ion (MnO4

Observation: the purple solution turns green.

Recognising species (observations):

Some species can not be recognised by their colour as they are colourless or do not have a distinct colour.Therefore you need to memorise some tests to detect these species:I2: turns blue/black with starch solutionSO4

2- : forms a white precipitate with H+/BaCl2

Fe3+ : forms a blood red solution with KSCNCl2 : turns damp starch-iodide paper blue/black

Recognising species (observations):Complete worksheet #2

Balancing complicated half equations

1. Balance the atoms that aren’t O or H.

Cr2O72- → Cr3+

Cr2O72- → 2Cr3+

2. Balance the oxygen by adding water.

Cr2O72- → 2Cr3+ + 7H2O

3. Balance the hydrogens by adding H+.

Cr2O72- + 14H+ → 2Cr3+ + 7H2O

4. Add electrons to the side that is more positive.

Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H2O

Balancing complicated half equations

1. Balance the atoms that aren’t O or H.

SO2 → SO42-

2. Balance the oxygen by adding water.

SO2 + 2H2O → SO42-

3. Balance the hydrogens by adding H+.

SO2 + 2H2O → SO42-+ 4H+

4. Add electrons to the side that is more positive.

SO2 + 2H2O → SO42-+ 4H+ + 2e-

Combining the half equations

The number of electrons in the two half equations needs to be the same.Therefore the half equations have to

be multiplied by a factor.

Combining the half equationsCr2O7

2- + 14H+ + 6e- → 2Cr3+ + 7H2O

SO2 + 2H2O → SO42-+ 4H+ + 2e-

Multiply whole equation by 3

3 SO2 + 6H2O → 3 SO42-+ 12H+ + 6e-

Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H2O

3 SO2 + 6H2O → 3 SO42-+ 12H+ + 6e-

Combining the half equations

Cr2O72- + 14H+ + 3 SO2 + 6H2O→2Cr3+ + 7H2O + 3 SO4

2-+ 12H+

5. Cancel out H+ and H2O.

Cr2O72- + 2H+ + 3 SO2 → 2Cr3+ + H2O + 3 SO4

Balancing redox equations in acidic/neutral conditions

Last year you learned the steps to balance redox reactions. One of the steps included adding H+ ions.H+ ions are present in small amounts in neutral conditions (same as OH- ions).In acidic conditions H+ ions are present in a higher concentration than OH- ions.It is therefore ok if your redox equations contain H+ ions as they are present.

Balancing redox equations in acidic/neutral conditions

The permanganate ion can be reduced in neutral, acidic and in alkaline conditions.Each of these conditions leads to a different product.Write down the balanced half equations for the reduction or permanganate in acidic conditions (producing Mn2+),

Answers

Acidic:MnO4

- + 8H+ + 5e- → Mn2+ + 4H2O

Now write a balanced half equation for the reduction of permanganate in neutral conditions (producing MnO2).

Answers

Neutral:MnO4

- + 4H+ + 3e- → MnO2+ 2H2OH+ ions are relatively scarce. Add OH- ions to both sides of the equation to neutralise.

MnO4- + 2H2O + 3e- → MnO2+ 4OH-

Balancing redox equations in alkaline conditions

The permanganate ion can also be reduced in alkaline conditions, producing the manganate ion (MnO4

2-).Write a balanced half equation using the usual steps.If adding H+ ions (which are scarce in the alkaline solution) is necessary, add an equal amount of OH- ions to both sides of the equation.

Answer

Alkaline:MnO4

- + e- → MnO42-

Complete worksheet #3

Key Learning Outcomes

You should now be able to:• Determine the oxidation number of any atom in a

compound or ion and use oxidation numbers to identify the oxidised and reduced species in a reaction.

• Recall common oxidising and reducing agents, state the colours of the reagents and their products, and recall any other observations or conditions characteristic of their use.

• Write ion-electron equations for oxidation and reduction half-reactions and combine the half equations to give a balanced ionic equation.

Redox Reactions

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